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Forget little green Martians invading Earth. Scientists are now concerned bacteria from Earth may be hitching rides to Mars on the many probes that have landed on the Red Planet.

According to NASA, the invading organisms "have the potential to contaminate other celestial bodies, making it difficult for scientists to determine whether a life form existed on another planet or was introduced there by explorers."

In order to reduce the potential of contamination, scientists say they need to better understand which kinds of organisms could survive the flight. Recent studies show that certain microbes are tougher than expected, with the ability to survive an interplanetary journey.

“If you are able to reduce the numbers to acceptable levels, a proxy for cleanliness, the assumption is that the life forms will not survive under harsh space conditions,” explains Kasthuri J. Venkateswaran, a researcher with the Biotechnology and Planetary Protection Group at NASA's Jet Propulsion Laboratory and a co-author of three papers on the topic in a statement.

NASA said spore-forming bacteria could present the greatest risk as they can "withstand certain sterilization procedures and may best be able to survive the harsh environments of outer space or planetary surfaces."

Spores from Bacillus pumilus SAFR-032 are proving to have the ability to resist the techniques to clean spacecraft, like ultraviolet radiation and peroxide treatment.

These spores, NASA said, were able to survive 30 minutes in a simulated Martian environment, whereas normal spores only survive 30 seconds.

Another test was done in space when the spores were exposed to outer space for 18 months aboard the European Technology Exposure Facility (EuTEF), a test facility mounted outside the International Space Station.

“After testing exposure to the simulated Mars environment, we wanted to see what would happen in real space, and EuTEF gave us the chance,” says Venkateswaran. “To our surprise, some of the spores survived for 18 months.”

NASA said the surviving spores showed increased levels of proteins associated with UV radiation resistance, as well as higher resistance when "revived and re-exposed on Earth."

Another experiment showed that the spores of Bacillus pumilus SAFR-032 and another spore-forming bacteria, Bacillus subtilis 168 were also very tough. They were dried on pieces of spacecraft and subjected for 1.5 years to the vacuum of space as well as various types of extraterrestrial radiation and temperature changes on the EuTEF.

The spores were also exposed to a simulated Martian atmosphere on EuTEF.

NASA said most of the organisms didn't survive, but that when UV rays were filtered out and the samples kept in the dark, 50 percent or more of the spores survived.

That, NASA says, "makes it likely that spores could survive a trip on a spacecraft to Mars if they are sheltered against solar radiation, perhaps in a tiny pocket of the spacecraft surface or underneath a layer of other spores."

A final study put rock-colonizing cellular organisms in the EuTEF for 1.5 years, to test if living organisms might move from one planet to another on rocks, a process known as lithopanspermia.

In another finding, organisms from Earth that live in extreme conditions proved they might be able to survive the arduous journey through space. However, they weren't able to replicate exact conditions as such a journey would possibly last up to millions of years.

The research, scientists hope, will lead to methods of minimizing the risk of microbial hitch hikers. Furthermore, they would help identify the kind of microbes that could originate on Earth.

U.S. Secretary of State John Kerry has told his Russian counterpart, Sergei Lavrov, that his country could face further consequences to what he called its “already strained economy” if Moscow does not fully comply with a cease-fire in Ukraine. The two met, on Monday, on the sidelines of a U.N. Human Rights Council meeting in Geneva, where Kerry outlined human rights violations in Russian-annexed Crimea and eastern Ukraine. VOA State Department correspondent Pam Dockins reports from Geneva.

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U.S. Secretary of State John Kerry has told his Russian counterpart, Sergei Lavrov, that his country could face further consequences to what he called its “already strained economy” if Moscow does not fully comply with a cease-fire in Ukraine. The two met, on Monday, on the sidelines of a U.N. Human Rights Council meeting in Geneva, where Kerry outlined human rights violations in Russian-annexed Crimea and eastern Ukraine. VOA State Department correspondent Pam Dockins reports from Geneva.

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